The term streaming media describes the playback of media on a playback device, where the media is stored on a server and continuously sent to the playback device over a network during playback. Typically, the playback device stores a sufficient quantity of media in a buffer at any given time during playback to prevent disruption of playback due to the playback device completing playback of all the buffered media prior to receipt of the next portion of media. Adaptive bitrate streaming or adaptive streaming involves detecting the present streaming conditions (e.g. the user's network bandwidth) in real time and adjusting the quality of the streamed media accordingly. Typically, the source media is encoded at multiple bit rates and the playback device or client switches between streaming the different encodings depending on available resources.
A common goal with adaptive bitrate streaming is to stream the highest bitrate stream available given the streaming conditions experienced by the playback device without stalls in the playback of media due to underflow. Underflow occurs when the playback device receives streaming media at a lower speed than the speed at which the media is played back. The video used in most adaptive bitrate streaming systems is encoded using variable bit rate encoding, which is typically more efficient. Even though the bitrate of the stream varies in time, the stream is typically described based upon its average bit rate. When variable bitrate encoding is used, the maximum bitrate of the stream is the rate that ensures no underflow will occur given a certain buffer size. Most playback devices accommodate variation in the size of the encoded frames using a buffer. In the context of video, the buffering delay (which can also be referred to as the seek delay) is the time a playback device must wait between starting filling the buffer and commencing playback to prevent underflow (i.e. a certain amount of data is buffered before decoding can start).